The present invention relates to a transparent conductive layer on the light incident side of a solar cell, a photosensor, or other photovoltaic element. The present invention relates, in particular, to those technologies which enhance the photoelectric conversion efficiency of photovoltaic elements and those technologies which increase long-term stability by inhibiting light degradation and heat degradation. The present invention relates also to improvements of cost-efficiency of photovoltaic elements.
Recently, many houses have been equipped with solar cells on their roof which are connected to the general power system so as to meet power requirements as much as possible. However, the power generation cost with solar cells is still high, which prohibits large-scale pervasion.
Although the employment of amorphous silicon-based thin-films as photovoltaic layers is said to be effective in improvement of the cost efficiency of solar cells, those thin-films have a low photoelectric conversion efficiency (conversion efficiency) as compared to crystalline solar cells and also suffer from decreases in the conversion efficiency when exposed to light irradiation, i.e., light degradation. With this, most of the past research so far pronounced on solar cells employing amorphous silicon-based thin-films, i.e., amorphous solar cells, has been concerned about two respects, i.e., xe2x80x9chow to increase the conversion efficiencyxe2x80x9d and xe2x80x9chow to decrease the light degradationxe2x80x9d.
So far, reportedly, transparent conductive layers could have been improved to obtain high-efficiency photovoltaic elements, i.e., solar cells. For example, according to Japanese Patent Application Laid-Open No. 8-77845, indium tin oxide (ITO) is formed and then exposed to a corpuscular beam of an inert gas to promote its own crystallization, thus obtaining low-resistivity, high-transmittance ITO thin-films. Also, according to another Japanese Patent Publication No. 7-84651, the crystallinity of ITO is controlled such that the  less than 111 greater than  axis orientates perpendicularly to the substrate surface, thus making a trigonometric cone of the ITO surface in geometry to decrease reflection loss and improve short-circuit current and also to improve the conversion efficiency. Also, according to another Japanese Patent Application Laid-Open No. 9-78236, ITO thin-films are formed with xenon gas instead of argon gas to enhance a carrier density, thereby obtaining low-resistance ITO thin-films at a relatively low substrate temperature.
Besides, an attempt has been made to form transparent thin-films into a stacked structure. For example, according to another Japanese Patent Publication No. 7-111482, transparent thin-films having different refractive indexes are stacked to obtain better reflection-preventing thin-films in a visible ray region of 450 to 650 nm. Those thin-films, however, are formed into a stacked structure comprising non-conductive thin-films.
According to another Japanese Patent Application Laid-Open No. 8-43840, a plurality of high-carrier-density thin-films (ITO:SnO2, 10% by weight (wt %)) and high-carrier-mobility thin-films (ITO:SnO2, 0.3 wt %) are stacked and annealed to obtain transparent conductive layers for LCD having a sheet resistivity of 5.4 xcexa9/xe2x96xa1.
Recently, on the other hand, such solar cells comprising single cells using xcexcc-Si:H thin-films as the i-type layer are reported that have a high conversion efficiency and no light degradation. This solar cell is attracting world attention as an alternative to such a type of a solar cell that employs an a-SiGe:H thin-film as the i-type layer. The xcexcc-Si:H thin-film eliminates light degradation inherent to amorphous silicon-based thin-films such as a-SiGe:H thin-films and, moreover, need not use costly material gases such as German gas (GeH4). In addition, this xcexcc-Si:H thin-film does not have a high absorption coefficient as compared to a-SiGe:H thin-films, but has a possibility to gain a short-circuit current (Jsc) almost like a-SiGe:H single cells by providing 2 xcexcm or larger thin-film thickness of the i-type layer. One example of reports to that effect is J. Meier, et al., xe2x80x9cOn the Way Towards High Efficiency Thin-Film Silicon Solar Cells by the Micromorph Conceptxe2x80x9d, MRS Symposium Proceeding, vol. 420, Amorphous Silicon Technology, pp. 3-13, 1996, where solar cells are reported that have the i-type layer comprised of microcrystalline silicon. This solar cell is made by VHF plasma enhanced CVD using a frequency of 110 MHz, achieving a conversion efficiency of 7.7% for single cells having one pin junction. This single cell has a great advantage of having no light degradation. Also, by further stacking thereon another pin junction containing amorphous silicon-based thin-film as an i-type layer to make a stacked cell, a conversion efficiency of 13.1% is attained. However, its light degradation rate is still high, almost the same as that for conventional amorphous silicon-based solar cells.
The inventors have made sure of such a time-wise phenomenon that indium tin oxide (ITO) thin-films have a higher resistivity with a rising temperature in the air. The inventors have also found that as light irradiation continues, a photovoltaic element using such ITO thin-films as transparent conductive layers has a higher temperature itself and a higher resistivity of the transparent conductive layer and also a lower fill factor, short-circuit light current, and conversion efficiency. For example, in an embodiment of Japanese Patent Application Laid-Open No. 8-56004, the vacuum evaporation method by use of electron beam is used to form transparent conductive layers comprised of ITO on a substrate. Also, in an embodiment of Japanese Patent Application Laid-Open No. 7-297428, the evaporation method is used to form transparent conductive layers comprised of ITO on a photovoltaic layer. Those photovoltaic elements having ITO thin-films formed by the vacuum evaporation method have a high initial conversion efficiency but, when exposed to intense light irradiation (e.g., 100 mW/cm2), has a higher resistivity of the ITO thin-films and a lower conversion efficiency as time passes. Also, according to Japanese Patent Application Laid-Open No. 8-43840, a plurality of high-carrier-concentration thin-films (ITO:SnO2, 10 wt %) and high-carrier-mobility thin-films (ITO:SnO2, 0.3 wt %) are stacked and then annealed, thereby obtaining low-resistivity transparent conductive layers for liquid crystal displays. However, when the above-mentioned thin-films are stacked on a photovoltaic layer and annealed, dopants such as phosphorus or boron are mutually diffused, thereby posing a problem of lowering open circuit voltage. Moreover, the light transmittancy (short-circuit current) was not high enough as required for photovoltaic elements.
In an example of Japanese Patent Application Laid-Open No. 6-5893, ITO is formed on a photovoltaic layer (pin layer) by the sputtering method. However, it suffers from a respect that the short-circuit current Jsc is small for single cells employing an a-SiGe:H thin-film as the i-type layer. As against this, research by the inventors has shown that a certain type of ITO thin-film formed by the sputtering method has a very high thermal stability, having a change rate of resistivity of approximately 1.1 even over a time lapse of 3000 hours at a temperature of 120xc2x0 C. The research by the inventors has shown also that a photovoltaic element having ITO thin-films formed on its photovoltaic layer by the sputtering method suffers from a respect that its short-circuit current is smaller than that of a photovoltaic element having ITO thin-films formed on its photovoltaic layer by the vacuum evaporation method. In addition, it has another problem that by the sputtering method, plasma comes in a high energy state to damage photovoltaic elements, thereby increasing the leakage current and decreasing the open circuit voltage. In an even worse case, it suffers from a respect that photovoltaic elements may be short-circuited. However, the photovoltaic element having ITO thin-films formed by the sputtering method has also advantages such as a high fill factor and a very excellent heat resistance.
One object of the present invention is to provide a photovoltaic element that has a high conversion efficiency which does not decrease so much even when exposed to intense light for a long time. Another object of the present invention is to eliminate the decrease in the conversion efficiency due to a rise in the temperature of a photovoltaic element caused by the irradiation of intense light.
Another object of the present invention is to improve the thermal stability of the conversion efficiency of photovoltaic elements having ITO thin-films formed on their photovoltaic layers.
As a means to solve the above-mentioned problems, the present invention provides a photovoltaic element having a p-type semiconductor layer and a transparent conductive layer comprised of indium tin oxide (ITO) junctioned face-to-face, wherein the transparent conductive layer comprises a plurality of layers and a sum of the content of tin oxide of the layer and that of tin which is the closest to the junction surface between the above-mentioned p-type semiconductor layer and transparent conductive layer of the plurality of layers is smaller than the sum of that for the other layers.
Indium tin oxide (ITO) contains mainly indium atoms, tin atoms, and oxygen atoms. Those indium atoms and tin atoms exist respectively in the state of indium oxide or another state such as an indium simple substance and the state of tin oxide or another state of a tin simple substance. The xe2x80x9csum of the content of tin oxide and that of tinxe2x80x9d as referred to in the present invention means, in terms of oxide, a sum of the mole concentration of tin oxide and that of tin present in the state of tin oxide such as a tin simple substance. In other words, the xe2x80x9csum of the tin oxide content and the tin contentxe2x80x9d means the value of a content of tin oxide calculated from an amount of tin atoms on the assumption that all of the tin exists in the state of tin oxide and all of the indium exists in indium oxide.
Such a value can be obtained by, for example, determining the concentration of tin atoms by the inductive coupling plasma emission (ICP) method, etc. and converting it in terms as oxide. If in this case the amount of tin present in a state other than tin oxide in the ITO is negligible, the xe2x80x9csum of tin oxide content and tin contentxe2x80x9d may be taken to be xe2x80x9ctin oxide contentxe2x80x9d.
In this case, it is preferable that the sum of the content of tin oxide and that of tin of a layer which is the closest to the above-mentioned junction surface be 10 mole % or less.
It is also preferable that the sum of tin oxide content and tin content for such a layer that has the largest sum of contents of tin oxide and tin of the above-mentioned plurality of layers be between 12 mole % and 30 mole %, both inclusive.
It is also preferable that the thickness of the layer closest to the above-mentioned junction surface be a half or less of the thickness of the entire transparent conductive layer.
It is also preferable that the thickness of the layer that has the largest sum of tin oxide content and tin content of the above-mentioned plurality of layers be a half or more of the thickness of the entire transparent conductive layer.
As another means to solve the above-mentioned problems, the present invention provides a photovoltaic element having a p-type semiconductor layer and a transparent conductive layer comprised of indium tin oxide (ITO) junctioned face-to-face, wherein the sum of the content of tin oxide and that of tin within the transparent conductive layer continuously changes in the film-thickness direction, coming in a minimum at the junction surface between the above-mentioned p-type semiconductor layer and transparent conductive layer.
In this case, it is preferable that the sum of a tin oxide content and a tin content within the transparent conductive layer on the above-mentioned junction surface be 10 mole % or less.
It is also preferable that the sum of a tin oxide content and a tin content of the region that has the largest above-mentioned sum of the above-mentioned plurality of layers be between 12 mole % and 30 mole %, both inclusive.
It is also preferable that the region whose sum of a tin oxide content and tin content in the above-mentioned transparent conductive layer is 10 mole % or less occupy a half or less of the entire transparent conductive layer.
It is also preferable that the region whose sum of tin oxide content and tin content in the above-mentioned transparent conductive layer is 12 mole % or larger occupy a half or more of the entire transparent conductive layer.
As another means to solve the above-mentioned problems, the present invention provides a photovoltaic element in a thermal equilibrium state having a p-type semiconductor layer and a transparent conductive layer comprised of indium tin oxide (ITO) junctioned face-to-face, wherein the lower end of the conduction band of the transparent conductive layer changes in the layer-thickness direction and also the difference between the level of the lower end of the conduction band of the transparent conductive layer in the vicinity of the junction surface between the p-type semiconductor layer and the transparent conductive layer and the Fermi level is larger than an average difference over the entire area of the transparent conductive layer.